1. Field of the Invention
This invention relates to the testing of underground formations or reservoirs and more particularly relates to determining formation pressure and formation permeability.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas from a subterranean formation, well boreholes are drilled into the formation by rotating a drill bit attached at a drill string end. The borehole extends into the formation to traverse one or more reservoirs containing the hydrocarbons typically termed formation fluid.
Commercial development of hydrocarbon fields requires significant amounts of capital. Before field development begins, operators desire to have as much data as possible in order to evaluate the reservoir for commercial viability. Various tests are performed on the formation and fluid, and the tests may be performed in situ. Surface tests may also be performed on formation and fluid samples retrieved from the well.
One type of formation test involves producing fluid from the reservoir, collecting samples, shutting-in the well and allowing the pressure to build-up to a static level. This sequence may be repeated several times at several different reservoirs within a given borehole. This type of test is known as a Pressure Build-up Test or drawdown test. One of the important aspects of the data collected during such a test is the pressure build-up information gathered after drawing the pressure down, hence the name drawdown test. From this data, information can be derived as to permeability, and size of the reservoir.
The permeability of an earth formation containing valuable resources such as liquid or gaseous hydrocarbons is a parameter of major significance to their economic production. These resources can be located by borehole logging to measure such parameters as the resistivity and porosity of the formation in the vicinity of a borehole traversing the formation. Such measurements enable porous zones to be identified and their water saturation (percentage of pore space occupied by water) to be estimated. A value of water saturation significantly less than one is taken as being indicative of the presence of hydrocarbons, and may also be used to estimate their quantity. However, this information alone is not necessarily adequate for a decision on whether the hydrocarbons are economically producible. The pore spaces containing the hydrocarbons may be isolated or only slightly interconnected, in which case the hydrocarbons will be unable to flow through the formation to the borehole. The ease with which fluids can flow through the formation, the permeability, should preferably exceed some threshold value to assure the economic feasibility of turning the borehole into a producing well. This threshold value may vary depending on such characteristics as the viscosity of the fluid. For example, a highly viscous oil will not flow easily in low permeability conditions and if water injection is to be used to promote production there may be a risk of premature water breakthrough at the producing well.
The permeability of a formation is not necessarily isotropic. In particular, the permeability of sedimentary rock in a generally horizontal direction (parallel to bedding planes of the rock) may be different from, and typically greater than, the value for flow in a generally vertical direction. This frequently arises from alternating horizontal layers consisting of large and small size formation particles such as different sized sand grains or clay. Where the permeability is strongly anisotropic, determining the existence and degree of the anisotropy is important to economic production of hydrocarbons.
A typical tool for measuring permeability includes a sealing element that is urged against the wall of a borehole to seal a portion of the wall or a section of annulus from the rest of the borehole annulus. In some tools a single port is exposed to the sealed wall or annulus and a drawdown test as described above is conducted. The tool is then moved to seal and test another location along the borehole path through the formation. In other tools, multiple ports exist on a single tool. The several ports are simultaneously used to test multiple points on the borehole wall or within one or more sealed annular sections.
The relationship between the formation pressure and the response to a pressure disturbance such as a drawdown test is difficult to measure. Consequently, a drawback of tools such as those described above is the inability to accurately measure the effect on formation pressure caused by the drawdown test.
In the case of the single port tool, the time required to reposition the port takes longer than time is required for the formation to stabilize. Therefore, the test at one point has almost no effect on a test at another point making correlation of data between the two points of little value. Also, the distance between the test points is now known to be critical in accurate measurement of the permeability. When a tool is moved to reposition the port, it is difficult to manage the distance between test points with the precision required for a valid measurement.
A multiple port tool is better than a single port tool in that the multiple ports help reduce the time required to test between two or more points. The continuing drawback of the above described multiple port tools is that the distance between ports is too large for accurate measurement.
The present invention addresses the drawbacks described above by providing an apparatus and method capable of engaging a borehole traversing a fluid-bearing formation to measure parameters of the formation and fluids contained therein.
An apparatus for determining a parameter of interest such as permeability of a subterranean formation is provided. The apparatus comprises a work string for conveying a tool into a well borehole, at least one selectively extendable member mounted on the work string. When extended, the at least one extendable member is in sealing engagement with the wall of the borehole and isolates a portion of the annular space between the work string and borehole. At least two ports in the work string are exposable to formation fluid in the isolated annular space. The distance between the ports is proportional to the radius of a control port to provide effective response measurement. A sensor operatively associated with each port is mounted in the work string for measuring at least one characteristic such as pressure of the fluid in the isolated section.
In addition to the apparatus provided, a method is provided for determining a parameter of interest of a subterranean formation in situ by conveying a work string into a well borehole. The work string and borehole have an annular space extending between the borehole and a wall of the borehole. At least one selectively extendable member is disposed on the work string for isolating a portion of the annulus. At least two ports are exposed to a fluid in the isolated annulus, and the at least two ports are separated from each other by a predetermined distance proportional to the size of at least one of the ports. A measuring device is used to determine at least one characteristic of the fluid in the isolated section indicative of the parameter of interest.
The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: